Digital imaging devices, using solid state image sensing devices, are widely used in medical imaging domain and industrial imaging domain. For example, digital X-ray detectors are converting X-rays to electrical signals for imaging inside subjects.
In general, a plurality of pixels in the solid state image sensing device have slightly different sensitivities for detection of light or X-ray. For improving output image quality the differences in sensitivity are corrected by a correction image which has information of the differences. The correction image will be acquired by an X-ray tube radiating X-rays to a X-ray detector with no subject between the X-ray tube and the X-ray detector.
For detectors whose input-output characteristics are not linear, Japanese patent application Laid Open JP2003-125203 discloses a calibration technique for acquiring multiple X-ray correction images. In each exposure for the multiple correction images the dose value increases at the same value. By using multiple correction images the differences in sensitivities of the plurality of pixels are corrected in a certain incoming dose range.
However, if the dose values of the multiple exposures increases at the same amount, the accuracy of approximation of nonlinear input-output characteristics may be relatively low, compared to the linear characteristics.